In rotary wing aircraft, the engines are typically mounted in the top part of the aircraft while the fuel tanks are typically located in the bottom part. During operation, the engine main fuel pump has to lift the fuel from the tank. Gravity and inertial forces acting on the fuel substantially reduce the pressure at the inlet of the engine mounted fuel pump below the fuel pressure in the tank, resulting in detrimental conditions for pump suction. The fuel pressure reduces even more when the aircraft flies at altitude, and the ambient air and tank pressures drop. The engine fuel boost pump has to possess exceptional suction capability to be able to induce the fuel from the inlet line at very low inlet pressures. In addition to this effect, due to rapid reduction in fuel pressure, the air, naturally dissolved in the fuel, evolves and travels toward the pump in form of air bubbles. Therefore, the fuel pump, in addition to its ability to induce the fuel at very low pressures, must also be able to induce air-fuel mixtures with high air content.
For some rotary wing aircraft applications, the inlet line geometry and the operating conditions act to separate air bubbles from the fuel stream creating a non-homogeneous mixture of air and fuel, which can be in the form of intermittent air bubbles or a relatively large bubble of air. For the boost pump to meet these air handling requirements, the boost pump must be able to compress air. Further, the boost pump must be incorporated into a fuel system that can store the compressed air bubble and can prevent it from reaching the inlet to the main fuel pump.
Another challenge for an engine mounted fuel pump in rotary wing aircraft is priming during initial engine start. There are several factors—such as initial (new) engine start, start following any engine fuel system component replacement, or minor leakage in the inlet fuel line—can cause the inlet fuel line to be empty of fuel prior the engine start. Many rotary wing aircraft do not incorporate any special equipment to fill the line with fuel and aid pump priming. Therefore, the engine mounted fuel pump has to be able to prime itself during engine start, including events when the inlet fuel line is empty.
Previous systems have applied centrifugal and regenerative impellers in the boost pump. Both of these impeller styles have limited to no ability to compress an air bubble at the inlet into a downstream cavity and are thus not able to pump non-homogeneous mixtures of air and fuel and are not self-priming.
Industrial applications, i.e. non-aircraft environments, have attempted to meet air pumping requirements by utilizing a side channel liquid ring pump. This type of pump is a hybrid that is able to provide pressures when operating on solid fuel that are on par with regenerative pumps but also has the capability to compress air.
When pumping air in a liquid ring pump, centrifugal forces separate the fuel and air (or vapor during low suction pressure conditions). The heavier fuel particles are flung to the outer diameter while the air bubbles collect near the impeller hub. A pressure gradient is established with the pressure in the channel at the outer diameter are greater than the pressure at the interior hub. The discharge port is located near the hub, away from the liquid ring.
The separation of air and fuel is not perfect and some fuel is lost out of the discharge port with the pumped air. As such, a fuel source is necessary to replenish the liquid ring pump to have stable performance. Otherwise, as fuel is depleted from the impeller cavity, the air compression will degrade during this priming operation. The side channel liquid ring pump is typically coupled with an inlet ejector to assist in suction conditions as well as to provide a conduit to replenish the liquid ring when pumping air during priming.
In industrial applications, to meet this need, the liquid ring pumps employ a large fuel reservoir at the discharge of the pump. The air and fuel mixture from the discharge port is separated by gravity in the reservoir. Air is pulled from the top of the reservoir and fuel is pulled from the bottom and then returned to the inlet of the pump to maintain the liquid ring. Unfortunately, this type of system is not practical for aerospace applications due to the large volume required for the air/fuel separator.
Embodiments of the present invention relate to improvements over the current state of the art.